What you downloaded was a "torrent" file. You
need a BitTorrent client, like BitTornado, to download the actual movie.

What's a torrent file?

It's sort of like a link on a web page. It has
the information BitTorrent needs to download and verify the file you
want.

How does it work?

You run BitTornado and use it to open the torrent
file. Inside the torrent file is a list of "tracker"
servers. BitTornado will ask the tracker for the addresses of
other BitTorrent clients who are downloading the same file you want; it
will then connect with them and you will start downloading from them.

It says I'm uploading
stuff from my computer. What is it uploading? I don't want
to upload stuff!

BitTorrent uploads pieces of the same data you are
in the process of downloading, and does not look anywhere else on your
computer for things to upload. It is designed to be very secure
that way. You can turn down the amount you upload, but this will
make your download slower.